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43 votes

What is the shape of orbit assuming gravity does not depend on distance?

Circular orbits are always possible for any central force law, but noncircular orbits would resemble rosettes. Here's a specific example for the case where the force is constant with distance: By ...
Sten's user avatar
  • 4,693
34 votes
Accepted

At what velocity would an object have to fall in order to become weightless?

There's no speed at which you experience weightlessness; to stay weightless, you need to keep getting faster and faster all the time. From our perspective on Earth, everything accelerates downward at ...
Darth Pseudonym's user avatar
22 votes

If the Earth had another moon would it be better protected from asteroids?

I don't quite buy JamesK's claim that "the moon only covers less than 0.001% of the sky, and so leaves us vulnerable to 99.999% of asteroids." That argument would work if typical asteroids 1)...
Ilmari Karonen's user avatar
16 votes

Does the escape velocity formula take into account how a gravitationally bound object's distance to its primary increases before coming back down?

The escape speed is defined in Newtonian physics simply by demanding that the sum of kinetic energy at launch (ballistically, with no power applied thereafter) and gravitational potential energy at ...
ProfRob's user avatar
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16 votes

Could the human body feel the sudden disappearance or end of a gravitational force?

Firstly, the sun can't just "disappear". Even if it were converted by magic into "pure energy", that energy can't go anywhere faster than the speed of light, and Energy has ...
James K's user avatar
  • 123k
14 votes

Gravitational forces

The centripetal acceleration from rotation is not that hard to calculate. It's pretty standard in engineering, usually for working out the forces on things that are spinning very fast, like propellers....
Darth Pseudonym's user avatar
12 votes

Concerning a binary system of stars/planets/black holes could one of them be ejected before eventually merging or colliding?

Not in Newtonian gravity with particles. This situation is soluble with stable elliptical orbits, so any examples would have to depend on either Relativity, or that the bodies are not particles. If ...
James K's user avatar
  • 123k
12 votes

Does the escape velocity formula take into account how a gravitationally bound object's distance to its primary increases before coming back down?

Yes, escape speed is an instantaneous calculation at the distance 'r' from the center of the object, as that changes you have to recalculate your escape speed. For example, these are the escape ...
Jason Goemaat's user avatar
12 votes

Is the gravitational constant really constant over our Universe?

See also Do the laws of physics work everywhere in the universe? on the Physics SE. If the gravitational constant were not constant, then the laws of physics work differently elsewhere in the universe....
Allure's user avatar
  • 4,554
11 votes

If the Earth had another moon would it be better protected from asteroids?

Neither effect is significant. Asteroids hit the Earth because they happen to be on a collision course with the Earth. The Earth's gravity can deflect some that might just miss the Earth onto a ...
James K's user avatar
  • 123k
9 votes
Accepted

Celestial "orbits"

Physical theories describe how things change in given circumstances (if the theory is right). In practice this means that they are applied in simplified ways, where the simplifications of ...
Anders Sandberg's user avatar
8 votes

What is the shape of orbit assuming gravity does not depend on distance?

In addition to Sten's great answer, it should be noted that under constant gravity all orbits are bounded. Therefore, there aren't any orbit like the hyperbolic ones mentioned in the question. That ...
Pere's user avatar
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8 votes

At what velocity would an object have to fall in order to become weightless?

Any speed. If you free fall then you feel weightless. That begins at zero speed, when you start falling, and ends when you hit something at arbitrarily high speed.
ProfRob's user avatar
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8 votes
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Gravity is inversely proportional to the distance squared, and tidal forces to the distance cubed. Is there any phenomenon inv/prop to the distance^4?

Yes, a very important one! I'm pretty sure the force field due to a rotating body's $J_2$ force field due to it's equatorial oblateness varies as $r^{-4}$. See Wikipedial's Geopotential model; Largest ...
uhoh's user avatar
  • 30.6k
8 votes

Why are black holes oblate spheroids?

By your logic that the gravity is infinite, you could equally ask how can a non-spinning black hole be a sphere rather than a point? The answer is that the surfaces you are discussing are not the ...
ProfRob's user avatar
  • 153k
7 votes
Accepted

Are there really confined Globular Clusters?

You are asking for a star cluster that sits at the bottom of an infinite potential well. Such clusters do not exist because they are largely the source of the potential that they reside in. There are ...
ProfRob's user avatar
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6 votes
Accepted

Dark Matter's effect on galaxy structure

The dark matter isn't directly responsible for the rotation of the Galaxy - that is a consequence of the initial angular momentum (or the angular momentum it has accrued during its formation). Dark ...
ProfRob's user avatar
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6 votes
Accepted

If you are in a deep gravity well, where time goes by more slowly, do you see the unfolding of a cosmic event at a different rate?

Time dilation is related to differences in gravitational potential in General Relativity. Observing a clock situated deep in a potential well, a distant observer would see it running slow. Vice-versa, ...
ProfRob's user avatar
  • 153k
6 votes

Can black holes even exist [if mass cannot be retained near the collapse threshold]?

A different way of looking at the issue: what properties would matter need to have to avoid imploding into a black hole if enough matter was gathered in a static sphere? The key thing about black ...
Anders Sandberg's user avatar
6 votes

Celestial "orbits"

You are right. But you miss a few quirks of reality, or some fine-print of the applicable physics. Yes, the Moon is moving away from Earth as slowing down Earth's spin rotation means that angular ...
planetmaker's user avatar
  • 19.5k
6 votes

At what velocity would an object have to fall in order to become weightless?

In general, you are weightless unless an electromagnetic force acts on you. (What we call "contact" with other matter like air or the ground is electromagnetic interaction of electron hulls. ...
Peter - Reinstate Monica's user avatar
5 votes
Accepted

Spiral Galaxies

You're right that a central mass can't explain the spiral structure (and you're also right about the tangent motion in case the string breaks). Spiral structure is indeed partially caused by gravity, ...
pela's user avatar
  • 38.5k
5 votes

Is it possible to detect gravitational lensing of both light and gravitational waves originating from the same event?

Yes; typically, if the light is lensed, the gravitational waves are also lensed, and vice versa. The main exception is if the lens is small. The current observatories, LIGO/Virgo/KAGRA, are sensitive ...
Sten's user avatar
  • 4,693
5 votes
Accepted

What is everything wrong with this theory of dark matter?

Sorry, but this is not correct. In the diagram, spacetime and hence the universe is the curved line. The sun, Earth, and any dark matter are not on spacetime, they are in it. Visualising spacetime as ...
James K's user avatar
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5 votes
Accepted

How to turn find velocity dispersion from radial velocity

The key assumption is that orbital velocity vectors are distributed randomly. Then the dispersion of velocities along each axis must be the same, e.g. $\sigma_x=\sigma_y=\sigma_z$ if $x$, $y$, and $z$ ...
Sten's user avatar
  • 4,693
5 votes

Gravitational waves vs. "normal gravity"

Acceleration isn't a property of spacetime per se. Two test particles at the same location can have different instantaneous accelerations, so just knowing that there is a passing gravitational wave ...
benrg's user avatar
  • 3,862
4 votes

Concerning a binary system of stars/planets/black holes could one of them be ejected before eventually merging or colliding?

Simple mass loss from a binary system can result in unbinding the system and one of the stars shooting off at relatively high speed. An example: Take a binary with a massive primary star, say $M_1=50 ...
ProfRob's user avatar
  • 153k
4 votes

Concerning a binary system of stars/planets/black holes could one of them be ejected before eventually merging or colliding?

When a binary star goes supernova, not only can they get separated, but one of them or both may be even ejected from the galaxy they are in. See https://www.nasa.gov/mission_pages/chandra/images/...
Thomas's user avatar
  • 3,494
4 votes

What is the shape of orbit assuming gravity does not depend on distance?

There wouldn't be standard orbits at all. The Earth orbits the Sun because the Sun's mass dominates the local spacetime. By removing distance from the gravitational equation, the Earth would, with ...
Michael Richardson's user avatar
4 votes
Accepted

Lack of objects between heliopause and Oort cloud?

There is no "gap" other than an apparent one caused by the use of logarithmic axes. The density of Oort cloud objects is thought to decrease with increasing distance from the Sun. However, ...
ProfRob's user avatar
  • 153k

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